Introduction

Acoustic neuromas are also called vestibular schwannoma, acoustic neurinoma, vestibular neuroma, and acoustic neurofibroma. These are tumors that evolve from Schwann cell sheath and can be either intracranial or extra-axial. They usually occur adjacent to the cochlear or vestibular nerve. Anatomically, acoustic neuromas tend to occupy the cerebellopontine angle. About 20% of internal carotid artery (ICA) tumors are meningiomas and may occur elsewhere in the brain. Bilateral acoustic neuromas also tend to be exclusively in individuals with type 2 neurofibromatosis.[1][2][3]

Etiology

Acoustic neuroma is allied to neurofibromatosis type 2 (defect on Chromosome 22) bilateral disease. Studies have shown that acoustic neuroma had a causative predisposition mutation. Radiation exposure may predispose a patient to the development of that condition as well.[4]

Epidemiology

Approximately 8% of all intracranial tumors which manifest clinically are attributed to schwannomas. The majority of acoustic neuromas are unilateral and sporadic. Bilateral acoustic neuromas are in fact genetic and constitute a bit less than 5% of all schwannomas. Acoustic neuromas in general, tend to present between the fourth to sixth decades of life. Acoustic neuromas developing in individuals with neurofibromatosis type 2 (NF II) are likely to present earlier, with a peak incidence around the third decade of life. Although rare, acoustic schwannomas can occur in children. There is a small female preponderance with an aggravation of problems during pregnancy. Hereditary acoustic neuroma occurs in NF II much more often compared to neurofibromatosis type 1 (NF I), although the latter is much more common. Unilateral acoustic neuroma has been reported exclusively in 24% of cases with NF I, while bilateral acoustic schwannoma is a hallmark of NF II. Both these conditions are autosomal dominant. The defective genetic locus has been localized to chromosome 17 in NF I and chromosome 22 in NF II.[5][6]

Histopathology

Grossly, the tumor is rubbery-firm with a pale, gray color. It shows different degrees of vascularity and has a well-defined capsule, which could be discoursed by the displaced and stretched nerve fibers. The cut section is usually pale gray and firm with a finely trabeculated appearance. Often, evidence of cystic degeneration, hemorrhage, xanthomatous changes and points of calcification may be present and are encountered in large tumors. These changes give a variegated appearance in consistency and color to the giant tumors. The blood supply of the tumor comes mainly from the internal auditory artery, which forks the surface of the tumor with several tiny branches. For larger tumors, there might be blood supply through small branches of the neighboring cerebellar and pontine arteries.

On light microscopy, the tumor is made up of spindle cells that have elongated nuclei and fibrillary cytoplasm. Those cells are arranged in two specific ways: Antoni A and Antoni B. Antoni A tissue is small, with a prominent, organized, interwoven course of elongated bipolar cells. Occasionally, the arrangement of the nuclei and fibers results in a spiral framework simulating that seen in meningiomas. Antoni B is represented by a random gathering of cells grouped around foci of cystic change, necrosis, old hemorrhage, and blood vessels. There is a variable amount of lymphocytic infiltration of this tissue. The Antoni B tissue type, generally seen in large tumors, is thought to be the outcome of ischemia. The consistency of the tumor is determined by the distant relative quantities of these two types. Although these are benign changes, as malignant transformation almost never happens, nuclear pleomorphism is typical in schwannomas. Mitotic figures are relatively rare. Necrosis, if found, is more due to a poor blood supply rather than fast development. Edema, development of micro or possibly macrocysts, xanthomatous alteration, and areas of calcification are attributed to the degenerative changes in the tumor tissue. Electron microscopy (EM) reveals the characteristic basement membrane of the Schwann cells as well as the presence of wide-spaced collagen.[7]

History and Physical

The signs and symptoms of acoustic neuroma are attributed to the involvement of the cranial nerve VIII, compressing surrounding cranial nerves, cerebellum, brainstem, as well as raised intracranial pressure (ICP). The majority of acoustic neuromas present with unilateral hearing loss due to cochlear nerve interruption or impairment of blood supply to the nerve. Other clinical features include tinnitus, decreased word understanding, vertigo, headaches, and facial numbness. With enough growth, the mass of CP angle will eventually compress the brainstem and cause gait abnormalities. The following is a summary of the detailed clinical features.

Due to the involvement of Cranial Nerve VIII

Auditory

Hearing impairment: the most common symptom, slowly progressive, high frequency retrocochlear sensorineural type. May pass unnoticed due to its insidious onset. May be examined in the physical through speech discrimination, using tuning forks of wide-range frequencies, Weber’s test as well as Rinne’s test.

Tinnitus: also a common symptom, can be intermittent

Vestibular: Instability while moving the head and nystagmus

Due to compression of other Cranial Nerves

Facial nerve: usually minimal with late presentation except for very large tumors. Depending on the degree of engagement of the nerve, the symptoms may include twitching, increased lacrimation and facial weakness

Trigeminal Nerve: paraesthesia in the trigeminal distribution, tingling of the tongue, impairment of the corneal reflex, and less commonly pain which may mimic typical trigeminal neuralgia

The glossopharyngeal and vagus nerves: palatal paresis, hoarseness of voice and dysphagia

Due to Cerebellar Compression

Occurs in large tumors

Symptoms include gait ataxia and incoordination of the upper limb, and rarely dysarthria

Evaluation

The diagnosis of an acoustic neuroma is made with a contrast MRI or a CT scan. Contrast is essential; otherwise, the non-enhanced scan can miss small tumors. If hearing impairment is present, audiometric tests are needed. Auditory brainstem evoked response is not used frequently to screen for acoustic tumors as it can miss small malignancies.[8][9][10]

Treatment / Management

Acoustic neuroma may be treated by:

Observation if small and in elderly patients with numerous comorbidities

Stereotactic radiotherapy or

Open craniotomy. The type of treatment depends on surgeon experience and patient preferences. After surgery, MRI is required within 6-12 months to document the extent of tumor removal and serve as a baseline. The most common complications of surgery include injury to the anterior inferior cerebral artery, hemorrhage, cerebellar trauma, facial paralysis, hearing loss (the most common)and hydrocephalus.

Differential Diagnosis

Acoustic neuroma accounts for around 80% to 90% of cerebellopontine angle (CPA) lesions. Differential diagnosis of an acoustic neuroma include:

Meningioma (5% to 10% of CPA lesions)

Ectodermal inclusion tumors

Epidermoid (5% to 7% of CPA lesions)

Dermoid

Metastases

Neuroma from cranial nerves other than cranial nerve VIII:

Trigeminal neuroma.

Facial nerve neuroma

Neurinoma of the lowest four cranial nerves (IX, X, XI, XII)

Arachnoid Cyst

Neurenteric Cyst

Cholesterol granuloma (distinct from epidermoid)

Aneurysm

Dolichobasilar ectasia

Extensions of nearby lesions in the CP angle:

Brainstem or cerebellar glioma

Pituitary adenoma

Craniopharyngioma

Chordoma and tumors of skull base

Fourth ventricle tumors (ependymoma, medulloblastoma)

Choroids plexus papilloma from the fourth ventricle through Foramen of Luschka

Glomus jugulare tumor

Tumors of temporal bone

Complications

Most complications are related to the surgery and include the following:

Injury to the anterior or posterior intracerebral arteries

Neurological injury

Brain herniation

Brain hemorrhage

Injury to the cerebellum

Facial paralysis

Hearing loss

Pearls and Other Issues

Recurrence is not common after removal, but tinnitus may worsen in some patients. Hearing loss and facial paralysis may improve with time.

The pre-operative distinction between a meningioma and an acoustic schwannoma is important for technical and prognostic reasons.

Enhancing Healthcare Team Outcomes

Acoustic neuromas are not common tumors in clinical practice. However, they often present with unilateral hearing loss and thus healthcare workers should consider the lesion in the differential diagnosis. The tumor once diagnosed is best managed by a multidisciplinary team that includes a neurosurgeon, neurologist, ENT surgeon, and a hearing specialist. After surgery, tinnitus is a common problem in at least 10-20% of patients. The recurrence rate after excision is less than 5%. Facial nerve paralysis appears to occur in about 15-30% of patients but most make a complete recovery over time. Hearing loss occurs in more than 50% of patients and may not improve over time. Residual hearing loss has a significant impact on the quality of life. [11][12][13] (Level V)

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The translabyrinthine approach can be used only for tumors less than 2 cm

The translabyrinthine approach permits early identification of the facial nerve in an area of the temporal bone where it has not been distorted by tumor growth

The translabyrinthine approach can be very difficult if the sigmoid sinus is very anteriorly placed or the jugular bulb is high

The translabyrinthine approach minimizes manipulation and retraction of the cerebellum

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Acoustic neuromas develop from the Schwann cells that invest the vestibular nerve and, therefore, are not genuine neural neoplasms

Acoustic neuromas are slow growing and have very predictable growth rates

Recent data show that acoustic neuromas develop from the inferior and superior vestibular nerves with equal frequency

Acoustic neuromas usually displace the facial nerve anteriorly

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A 65-year-old man complains of unsteadiness and vertigo with head motion after an acoustic neuroma is removed from his left ear. In consideration of vestibular rehabilitation, what should the appropriate therapy involve?

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Drooling and facial weakness is a common exam finding with large tumors

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A patient has a one-week history of vertigo, tinnitus, dizziness, and right-sided gait ataxia. Electronystagmography or calorics using cold-water in the left ear cause right-beating nystagmus, and warm water causes left-beating nystagmus. There is no response to cold caloric testing of the right ear. Select the most likely diagnosis.

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